Steam Generator for Combined Cycle Gas Turbine Plant

ABSTRACT

A steam generator for the generation of steam from recovered heat from the flue gases of the gas cycle of a combined cycle gas turbine power plant comprising at least one high pressure drum; at least one intermediate pressure drum; at least one low pressure drum; steam outlet means to enable extraction of auxiliary process steam from a location downstream of at least one drum. A combined cycle gas turbine power plant including the above.

The field of the invention relates to an improved method of steam extraction from a combined cycle gas turbine power plant to provide a steam source for various auxiliary power and thermal requirements, and in particular at least for use in an associated post combustion carbon capture plant. The invention in particular relates to a steam generator for the generation of steam from recovered heat from the flue gases of the gas cycle of a combined cycle gas turbine power plant, especially adapted for use with an associated post-combustion carbon capture apparatus, and to a combined cycle gas turbine power plant incorporating the same.

Most of the energy used in the world today is derived from the combustion of fossil fuels, such as coal, oil, and natural gas. Post-combustion carbon capture (PCC) is a means of mitigating the effects of fossil fuel combustion emissions by capturing CO₂ from large sources of emission such as thermal power plants which use fossil fuel combustion as the power source. The CO₂ is not vented to atmosphere but is removed from flue gases by a suitable absorber and stored away from the atmosphere. Other industrial processes where similar principles might be applicable to capture post-process CO₂ might include removal of CO₂ generated in a process cycle, for example removal of CO₂ from the process flow during production of ammonia, removal of CO₂ from a natural gas supply etc.

It is known that CO₂ can be separated from a gas phase, for example being the flue gas of a thermal power plant, by means of absorption by suitable absorption medium, for example absorbent in liquid phase, typically in aqueous solution. Gas is passed through the absorption medium under conditions of pressure and temperature optimised for removal of substantially all the carbon dioxide. The purified gas is then directed for further processing as necessary. The absorption medium rich in CO₂ is subjected to a stripping process to remove the CO₂ and regenerate the absorption medium.

Typically this process involves regenerative heating of the medium. The CO₂ rich medium is maintained at high temperature, which may be at or near boiling point of an absorbent liquid phase under pressure. The heat necessary is typically obtained when the system is used in association with a thermal power plant by supplying steam from the LP turbine system. At higher temperatures the medium will release the absorbed CO₂. Regenerated medium may be drawn off for reuse. The released CO₂ may then be collected for example for sequestration. The condensate product of the steam used to supply regenerative heat is returned to the steam generation system.

A conventional coal fired boiler has two main pressure levels for steam generation; the main steam pressure and hot reheat pressure. Even for coal fired boilers with a second reheat pressure, the pressure level is substantially higher than those prevailing at low pressure turbine inlet, which is compatible with the pressures having saturation temperatures corresponding to the upper limits of solvent operation.

Steam extraction for solvent regeneration in an associated post combustion carbon capture plant (and/ or for other auxiliary requirements) cannot therefore be taken from the steam generator and has to be taken from the IP/LP cross-over. This is used for supplying energy for solvent regeneration utilising latent heat. However, this steam extraction reduces the inlet mass flow to the low pressure steam turbine and reduces the power output together with affecting its operational integrity.

A combined cycle gas turbine power plant employs both a gas cycle and a steam cycle for power generation, and has a steam cycle steam generator for the generation of steam from the waste heat in the flue gases of the gas cycle. Such a steam generator is conventionally known as a heat recovery steam generator (HRSG) or waste heat recovery boiler (WHRB).

A location has to be determined for extracting process steam at an optimum pressure level for various uses in particular in a post combustion carbon capture plant.

The ideal tap-off location should be consistent with prevalent manufacturing limits and ranges for conventional equipment used in power plants (particularly, combined cycle gas turbine power plants) and avoid the energy penalty incurred in extracting steam from the IP/LP cross-over, which simultaneously affects operational integrity of the low pressure steam turbine.

In the particular case, a suitable location for condensate return from post combustion carbon capture plants should also be determined involving least modification of conventional thermal cycles.

In accordance with the invention a steam generator for the generation of steam from recovered heat from the flue gases of the gas cycle of a combined cycle gas turbine power plant comprises:

at least one high pressure drum;

at least one intermediate pressure drum;

at least one low pressure drum;

steam outlet means to enable extraction of auxiliary process steam from a location downstream of at least one of the said drums to supply an associated post combustion carbon capture plant with a source of motive power and/or latent heat.

The steam generator will typically be a heat recovery steam generator (HRSG) or waste heat recovery boiler (WHRB) of generally familiar design, in particular comprising HP, IP and LP modules for the generation of steam from recovered heat from the flue gases of the gas cycle of a combined cycle gas turbine power plant. It is distinctly characterised in the provision of a tap off location for auxiliary process steam which is located within the HRSG.

Process steam is drawn off to supply additional process module(s) of the combined cycle gas turbine power plant with a source of motive power and/or latent heat. The steam generator conveniently further comprises a fluid conduit fluidly continuous with the steam outlet means to convey auxiliary process steam to such additional process module(s).

In accordance with the invention, process steam is drawn off to supply at least an associated post combustion carbon capture plant with a source of motive power and/ or latent heat. The steam generator conveniently further comprises a fluid conduit fluidly continuous with the steam outlet means to convey auxiliary process steam to an associated post combustion carbon capture plant.

In a more complete embodiment of the invention, it follows that the steam generator as above described is provided in a combined cycle gas turbine power plant in combination with at least one gas cycle generator and/ or a post-combustion carbon capture apparatus. In a more complete embodiment, it follows that a combined cycle gas turbine power plant comprises:

at least one gas cycle generator;

a steam generator for the generation of steam from recovered heat from the flue gases of the at least one gas cycle generator comprising:

at least one high pressure drum;

at least one intermediate pressure drum;

at least one low pressure drum;

steam outlet means to enable extraction of auxiliary process steam from a location downstream of at least one of the said drums;

a post-combustion carbon capture apparatus fluidly disposed to recover CO₂ from combustion gases generated by the power plant in use;

a fluid conduit fluidly continuous with the steam outlet means to convey auxiliary process steam to the post-combustion carbon capture apparatus.

The invention is distinctly characterised in that auxiliary process steam is extracted from the recovery steam generator of a combined cycle gas turbine power plant and not from the turbine. Steam outlet means are provided to enable extraction of auxiliary process steam from one or more outlet locations in the recovery steam generator. Auxiliary outlet locations may be provided downstream of the at least one high pressure drum and/or of the at least one intermediate pressure drum and/ or of the at least one low pressure drum.

The auxiliary process steam may be used in particular to regenerate lean absorbent in a post-combustion carbon capture apparatus using absorbent medium such as absorbent solution, for example to supply latent heat to a re-boiler for lean solvent regeneration. Other supplies of auxiliary process steam may be made available for example to the steam turbine auxiliary drive, and to the reclaimer for maintaining solvent properties.

The person skilled in the art will be familiar with suitable cycle gas turbine power plant and PCC systems. The invention is not particularly limited to any such system.

The post-combustion carbon capture apparatus may for example comprise an absorber column where CO₂ is separated from the flue gas by means of absorption by passing the gas through a column where the gas flows in an opposite direction to an absorbent in liquid phase, typically in aqueous solution.

The post-combustion carbon capture plant apparatus for example further comprise a regeneration column where CO₂ is removed from absorbent by regenerative heating, for example to at or near boiling point of an absorbent liquid phase under pressure. A suitable heating means is for example a condenser reboiler as is familiar. Again, as is familiar, this condenser reboiler is disposed to receive solution that has passed through a process volume, for example via an outlet towards the bottom of a column, and reboil the solution to regenerate lean absorbent.

Conveniently, auxiliary process steam is used to supply latent thermal energy for the above processes. It is a particular advantage of the invention that the thermal energy of the resultant condensate can be more effectively recovered.

The location chosen for tap off of auxiliary process steam is motivated by the following considerations in particular in relation to the preferred case where process steam is supplied to a post-combustion carbon capture apparatus:

to minimise steam consumption by utilising the latent heat at the maximum corresponding saturation temperature essential for the maintenance of solvent properties; for the regeneration of the lean solvent subsequent to the stripping of the rich solvent from carbon;

to provide a steam source for providing motive power to a steam turbine for generating power for pumps, fans and compressors in the post combustion carbon capture plant;

to provide a source of steam for providing thermal energy for the reclaimer used for the regeneration of the solvent for absorbing carbon.

In a preferred case at least one outlet location is provided downstream of the at least one low pressure drum. Examples discussed below make use of this location. However, auxiliary process steam is not necessarily taken or taken only from the low pressure drum. Depending on factors such as drum size and steam demand it might be desirable to take auxiliary process steam additionally or alternatively from the intermediate pressure and/or high pressure drums, and this can be done without departing from the general principles of the invention. The examples below will be understood accordingly.

The concept of the invention provides a working schematic for a post combustion carbon capture scheme for a combined cycle gas turbine plant which does not imply any energy penalty from the main power producing components, the gas turbine and the steam turbine, and provides a methodology for utilising the third extraction point from the heat recovery steam generator or waste heat recovery boiler, which was conventionally used for providing steam to the low pressure steam turbine. This avoids the problem known in conventional coal fired boilers with single reheat involving steam generation at two pressures which necessitates the tap-off for extraction steam for the post combustion carbon capture plant from the IP/LP cross-over reducing steam inlet mass flow to the low pressure steam turbine, substantially reducing power output and affecting operational integrity.

The condensate from the post combustion carbon capture plant is preferably returned by auxiliary condensate pumps back to the feed water storage tank, which is the source of providing the working fluid to the low pressure economiser upstream of the low pressure steam drum.

This implies a marginal capacity addition to the sizing of the deaerator, the feed pump to the economiser and the low pressure steam circuit in the heat recovery steam generator or waste heat recovery boiler, with a check on dew point.

However, the major part of designing the high pressure and intermediate pressure circuit of the heat recovery steam generator or waste heat recovery boiler remains unaffected.

In particular in consequence the invention is suitable for both green field projects and retro-fitting on a brown field project if the LP circuit and the Feed Water Storage Tank of the HRSG/WHRB can be upgraded to conform to the operational requirement of the post combustion carbon capture process. Substantial parts of existing plants can be left untouched with no necessity for modification. In consequence in the preferred case this aspect of the invention comprises a method of after-market modification of existing plant in situ.

The invention will now be described by way of example only with reference to FIG. 1 of the accompanying drawings, which is a schematic of an embodiment of a system in accordance with the invention.

The FIGURE describes the water-steam flow schematic of the low pressure (LP), intermediate pressure (IP) and high pressure (HP) circuit of the heat recovery steam generator (HRSG) or waste heat recovery boiler (WHRB) against the direction of flue gas flow from gas turbine, through an arrangement of drums, economisers (ECO), evaporators (Eva) and super-heaters (SH).

Main Steam (MS) is supplied from HP SH to HP turbine, the cold reheat (CRH) steam is reheated to the hot reheat (HRH) steam for maintaining pressure in Feed Water Storage Tank and deaeration and supplying steam to the IP turbine. LP Steam is supplied from LP SH to LP Turbine and components of the post combustion carbon capture plant—namely the steam turbine auxiliary drive, re-boiler for lean solvent regeneration and reclaimer for maintaining solver properties.

The condensate is returned after condensation from condensate extraction pumps into the feed water storage tank.

The invention provides a scheme of operation of a post combustion carbon capture process in a combined cycle gas turbine plant without incurring any energy penalty from rated electric output and operational inadequacy of steam turbine.

The invention can be implemented with the minimum modifications to conventional design principles of heat recovery steam generator or waste heat recovery boiler, leaving the high pressure and intermediate pressure circuits intact.

The operational stability of the post combustion carbon capture process becomes essentially related to level control in the feed water storage tank and control and protection logic for the LP circuit; together with back-pressure maintenance for the gas turbine.

The operational stability of the post combustion carbon capture process is uncoupled from the power production process of the steam turbine. 

1. A steam generator for the generation of steam from recovered heat from the flue gases of the gas cycle of a combined cycle gas turbine power plant comprising: at least one high pressure drum; at least one intermediate pressure drum; at least one low pressure drum; steam outlet means to enable extraction of auxiliary process steam from a location downstream of at least one of the said drums to supply an associated post combustion carbon capture plant with a source of motive power and/ or latent heat.
 2. A steam generator in accordance with claim 1 wherein the steam generator further comprises a fluid conduit fluidly continuous with the steam outlet means to convey auxiliary process steam to an associated post combustion carbon capture plant.
 3. A steam generator in accordance with claim 1 adapted such that in use process steam is drawn off to supply additional process module(s) of the combined cycle gas turbine power plant with a source of motive power and/or latent heat in that the steam generator further comprises a fluid conduit fluidly continuous with the steam outlet means to convey auxiliary process steam to such additional process module(s).
 4. A steam generator in accordance with claim 1 wherein at least one steam outlet means is provided downstream of the at least one low pressure drum.
 5. A combined cycle gas turbine power plant comprising: at least one gas cycle generator; a steam generator for the generation of steam from recovered heat from the flue gases of the gas cycle of a combined cycle gas turbine power plant comprising: at least one high pressure drum; at least one intermediate pressure drum; at least one low pressure drum; steam outlet means to enable extraction of auxiliary process steam from a location downstream of the at least one of the said drums; a post-combustion carbon capture apparatus fluidly disposed to recover CO₂ from combustion gases generated by the power plant in use; a fluid conduit fluidly continuous with the steam outlet means to convey auxiliary process steam to the post-combustion carbon capture apparatus.
 6. A plant in accordance with claim 5 wherein at least one steam outlet means is provided downstream of the at least one low pressure drum.
 7. A plant in accordance with claim 5 wherein the post-combustion carbon capture system comprises an absorber column where CO₂ is separated from the flue gas by means of absorption by passing the gas through a column where the gas flows in an opposite direction to an absorbent in liquid phase.
 8. A plant in accordance with claim 5 wherein the post-combustion carbon capture system further comprises a regeneration column where CO₂ is removed from absorbent by regenerative heating.
 9. A plant in accordance with claim 8 comprising a condenser reboiler is disposed to receive absorbent solution and reboil the solution to regenerate lean absorbent. 